This invention relates in general to polyimide resins and, more specifically, to simplified methods for making resilient, flame resistant modified polyimide and polyimide-amide foams.
Prior U.S. Pat. Nos. 4,161,477, 4,183,838 and 4,183,839 disclosed and claimed certain polyimide compositions which are flame resistant and useful as coatings and adhesives.
The coating and adhesive compositions described in the above-mentioned prior patents are made by first preparing a suitable bisimide by reacting an aromatic tetracarboxylic acid dianhydride with a cyclic amide or oxoimine. The ratio of oxoimine to dianhydride is preferably in the 2.3:1 to 2.7:1 range and the imidization reaction is preferably conducted at a temperature of 170.degree.-200.degree. C. for 20-60 minutes.
The polyimide forming material is then prepared by dissolving the bisimide in an inert solvent; then adding thereto a suitable diamine, producing a viscous fluid containing an intimate, unpolymerized mixture of N-substituted cyclic bisimide dicarboxylic acid and diamine which is capable of being converted to a high molecular weight polymer by the application of heat.
The solution is coated onto a surface and polymerized by heating to a temperature in the 177.degree.-316.degree. C. range for 30 minutes to 5 hours. The following is exemplary of the exchange reaction which occurs: ##STR1## where n is a positive integer and x is 2-7.
The resulting coating is tough, highly adherent to various surfaces, with very few pinholes or bubbles. It has excellent peel strength and is resistant to high temperatures, peeling and abrasion.
The prior coating material, however, was not suitable for use in applications requiring a cellular or foam material, since conventional agitation foaming and addition of known blowing agents add to process coats and complexity and are not entirely effective at the relatively high polymerization temperature required.
A method of producing polyimide foams which overcomes many of these problems is described in our earlier U.S. Pat. Nos. 4,394,464 and 4,426,463. In that method, an aromatic dianhydride was reacted with an oxoimine at a temperature of about 150.degree. C. to 200.degree. C. to produce an N-substituted aliphatic imide. The resulting product was cooled below about 70.degree. C. and dissolved in a reactive solvent esterifying agent and heated to reflux for at least 60 minutes to esterify the imide. The excellent, flame resistant, resilient foam could then be produced by heating the dry material to at least about 120.degree. C. for a suitable period.
While our prior method produced excellent polyimide foams, that method is undesirably complex, requiring a number of sequential steps to be performed at varying, relatively high, temperatures. This tended to result in high energy costs and varying product characteristics due to processing variations.